Rotary Cement Kiln Simulator (RoCKS): Integrated modeling of pre-heater, calciner, kiln and clinker cooler

Abstract

This paper presents an integrated reaction engineering based mathematical model for clinker formation in cement industry. Separate models for pre-heater, calciner, rotary kiln and cooler were initially developed and coupled together to build an integrated simulator. Appropriate models for simulating gas–solid contact and heat transfer in pre-heaters were developed. Calciner was modeled by considering simultaneous combustion of coal particles and calcination of raw meal. Complex heat transfer and reactions (solid–solid, gas–solid and homogeneous reactions in gas phase) in rotary kiln were modeled using three sub-models coupled to each other. Solid–solid reactions in the bed region of the kiln were modeled using pseudo-homogeneous approximation. Melting of solids in the bed and formation of coating within the kiln were accounted. Clinker cooler was simulated by developing a two-dimensional model to capture cross-flow heat transfer between air and hot clinkers. The individual models were coupled with each other via mass and energy communication through common boundaries. The coupled model equations were solved iteratively. The model predictions agree well with the observations and experience from cement industry. The model was used to gain better understanding of influence of operating conditions on energy consumption in cement plant. Several ways for reducing energy consumption were computationally investigated. The integrated model, the developed software RoCKS (for Rotary Cement Kiln Simulator) and results presented here will be useful for enhancing our understanding and for enhancing the performance of clinker manufacturing.

Introduction

Cement making processes are extremely energy consuming. Typically for producing one ton of cement, a well-equipped plant consumes nearly 3 GJ. For each ton of clinker produced, an equivalent amount of green house gases are emitted. The manufacture of cement has been the focus of considerable attention worldwide because of the high energy usage and high environmental impact of the process. Considering the recent impetus on reduction in emission of green house gases and reduction in energy consumption, there is a renewed emphasis on developing computational models for cement industry and using this understanding for performance enhancement.

A schematic of typical clinker making process is shown in Fig. 1. The raw meal consisting of predetermined quantities of CaCO3, SiO2, Al2O3 and Fe2O3 are passed sequentially through pre-heater, calciner, kiln and cooler to form cement clinkers. In a pre-heater section the raw meal is pre-heated to calcination temperature via hot gases coming from calciner. In a calciner, raw meal is partially calcined. The energy required for endothermic calcination reaction is provided by combusting a suitable fuel. In most cases, coal is used to provide the required energy, especially in India. The calciner is supplied with tertiary air from the cooler and air coming out of kiln exhaust. The former is to supply sufficient O₂ for coal combustion and later to utilize the heat of kiln gases to enhance calcination reaction. The hot gases from calciner are sent to pre-heater assembly for pre-heating the solids. The partially calcined solids from the calciner are fed slowly to a rotary kiln. In the rotary kiln, remaining calcination and other clinkerization reactions occur (formation of C₂S, C₃A, C₄AF). The energy required for endothermic clinker reactions is provided by combusting coal in the kiln. The pulverized coal along with the pre-heated air (secondary air) is fed to the kiln in a counter current mode with respect to solids. Part of the solids melts in the kiln. The melt formation causes an internal coating on kiln refractories. Counter current flow of gas entrains solid particles in the free board region. Such entrainment enhances rates of radiative heat transfer by increasing effective emissivity and conductivity. The hot clinkers are discharged from kiln to clinker cooler and hot gases from kiln exhaust are sent to the calciner. In a clinker cooler, a part of energy of solids is recovered back by heat exchange with air. The pre-heated air from the coolers is passed to kiln and calciner as secondary and tertiary air, respectively. A small part of air may be vented if required.

This brief overview of clinker formation clearly demonstrates the strong coupling among pre-heater, calciner, kiln and cooler. It is therefore essential to develop an integrated model for pre-heater, calciner, kiln and cooler in order to capture key characteristics of clinker manufacturing and to enable the model to be used as simulation or optimization tool. Such an attempt is made in this work.

Recently some attempts have been made to develop computational fluid dynamics (CFD) based models to simulate either calciner (for example, Lu et al., 2004) or kiln (for example, Mastorakos et al., 1999, Mujumdar and Ranade , 2003). Though such CFD models show promise in simulating details of combustion and burner designs, it is almost impossible to build CFD models for simultaneous and coupled simulations of pre-heaters, calciner, kiln and cooler. The CFD models are thus not very useful to gain understanding of coupling and exploring ways to reduce overall energy consumption per ton of clinker. Some attempts have also been made to develop reaction–engineering models for kiln (for example, Mujumdar and Ranade, 2006, Spang, 1972). Such models have shown promising capabilities in capturing the overall behavior and providing useful clues for reducing energy consumption in rotary cement kilns. The numerical experiments using the computational model could also predict the influence of kiln operating parameters on net energy consumption (NEC) in kilns. Such guidelines can provide useful hints to operating engineers for kiln optimization. However, none of these models have included coupling of pre-heater, calciner, kiln and clinker cooler. This work was undertaken to fulfill this need. The motivation of the present work was to develop a framework of reaction engineering based computational model for clinker formation in cement industry and use this framework subsequently for exploring possible performance enhancement. The paper is organized as follows.

The key issues in modeling individual models are discussed in Section 2. The computational model and the modeling strategy are thereafter presented in Section 3. Section 4 reports the results of computational simulations of model with respect to key operating parameters. The use of the developed model to explore possible ways of reducing energy consumption in kiln is discussed in Section 5. Key findings of the study are summarized at the end.